1. Technical Field
The present invention relates to a joint structure and a power steering device.
2. Related Art
Electric power steering devices are known to use electric motors to assist (aid) steering force. Known types of assisting methods include pinion-assist type, double pinion-assist type, rack-assist type, and column-assist type.
For example, the pinion-assist type includes: a worm wheel fixed to a pinion shaft; and a worm engaged with the worm wheel and rotatable by a motor. A configuration oftentimes used to connect the output shaft of the motor (motor shaft) and the worm to each other is such a configuration that a coupling integrally molded or press-fitted to the motor shaft and having a spline hole is spline-connected to a spline shaft portion molded integrally to the worm. In order to facilitate the spline connection in this configuration, a proper gap (space) is formed between the spline hole and the spline shaft portion.
The gap, however, can cause a backlash to occur between the coupling and the spline shaft portion. Specifically, there is a possibility of the coupling and the spline shaft portion wobbling in a circumferential direction (rotating direction), causing rattling noise (tapping noise) to be generated at the time when the motor starts rotation or reverses rotation. Additionally, at the time when the motor starts rotation or reverses rotation, there is an abrupt increase in torque that is being transmitted, causing the assisting force to fluctuate. This presents a possibility of a driver feeling uncomfortable.
In order to prevent rattling noise from occurring and prevent an abrupt increase in the torque, a joint that includes an elastic member is proposed (see patent literature 1 (JP-A-2005-306141) and patent literature 2 (JP-A-2005-212623)). A specific method is to sandwich the elastic member between metal members such as couplings and to transmit the torque while deforming the elastic member under compressive force.